During the process of phagocytosis, the polymorpho-nuclear leucocytes in the circulating blood increase greatly in numbers (_leucocytosis_), as well as in their phagocytic action, and in the course of destroying the bacteria they produce certain ferments which enter the blood serum. These are known as _opsonins_ or _alexins_, and they act on the bacteria by a process comparable to narcotisation, and render them an easy prey for the phagocytes. _Artificial or Passive Immunity._--A form of immunity can be induced by the introduction of protective substances obtained from an animal which has been actively immunised. The process by which passive immunity is acquired depends upon the fact that as a result of the reaction between the specific virus of a particular disease (the _antigen_) and the tissues of the animal attacked, certain substances--_antibodies_--are produced, which when transferred to the body of a susceptible animal protect it against that disease. The most important of these antibodies are the _antitoxins_. From the study of the processes by which immunity is secured against the effects of bacterial action the serum and vaccine methods of treating certain infective diseases have been evolved. The _serum treatment_ is designed to furnish the patient with a sufficiency of antibodies to neutralise the infection. The anti-diphtheritic and the anti-tetanic act by neutralising the specific toxins of the disease--_antitoxic serums_; the anti-streptcoccic and the serum for anthrax act upon the bacteria--_anti-bacterial serums_. A _polyvalent_ serum, that is, one derived from an animal which has been immunised by numerous strains of the organism derived from various sources, is much more efficacious than when a single strain has been used. _Clinical Use of Serums._--Every precaution must be taken to prevent organismal contamination of the serum or of the apparatus by means of which it is injected. Syringes are so made that they can be sterilised by boiling. The best situations for injection are under the skin of the abdomen, the thorax, or the buttock, and the skin should be purified at the seat of puncture. If the bulk of the full dose is large, it should be divided and injected into different parts of the body, not more than 20 c.c. being injected at one place. The serum may be introduced directly into a vein, or into the spinal canal, _e.g._ anti-tetanic serum. The immunity produced by injections of antitoxic sera lasts only for a comparatively short time, seldom longer than a few weeks. _"Serum Disease" and Anaphylaxis._--It is to be borne in mind that some patients exhibit a supersensitiveness with regard to protective sera, an injection being followed in a few days by the appearance of an urticarial or erythematous rash, pain and swelling of the joints, and a variable degree of fever. These symptoms, to which the name _serum disease_ is applied, usually disappear in the course of a few days.